The goal of the proposed research is to understand the processes that control calcium entry in pinched-off presynaptic nerve terminals (synaptosomes) from rat brain. Synaptosomes will be utilized, because they retain in vitro many of the functional properties of intact neuronal tissue. Two voltage-regulated pathways that are selective for divalent cations, probably Ca channels, have been detected. They are pharmacologically and functionally distinct. They will be monitored by measuring influx of 45Ca and other radioactive tracers (e.g., 85Sr, 133Ba, 28Mg, 22Na). A rapid filtration technique will be used to accurately resolve the time-course of influx. The pathways for Ca entry will be characterized with respect to ionic selectivity, binding sites for polyvalent cations, reactive amino-acid groups, and interactions with several centrally acting drugs. The anticonvulsant diphenylhdantoin (phenytoin), barbiturates and opiates will be examined. These drugs have been reported to reduce Ca uptake in nerve endings, but it is not known how they produce this effect. A rapid-mixing apparatus will be employed to study inactivation of the Ca entry pathway in synaptosomes.